Conventional froth flotation is not efficient to separate coarse particles. In industrial practice, sulphide particles, for example, larger than about 0.3 mm (a weight of the order of 0.1 mg) are very difficult to recover. In non-metallic flotation, under the most favourable conditions, the upper limit of froth flotation is about 1 mm which corresponds to particle weight of 1-2 mg.
A limited number of studies has been devoted to the problem of coarse particle flotation. Most researches have been limited to the problem of beneficiation of very fine particles and there is little substantial understanding of the flotation behavior of coarse particles. Obviously, the low floatability of large particles is somehow related to the extra weight that has to be lifted to the surface (usually under highly turbulent conditions) and then transferred and maintained in the froth layer. Factors, such as density of the solid, turbulence, stability and height of the froth layer, tenacity of the particle-bubble attachment, depth of the water column, and other variables that can indirectly influence the factors listed above, determine the floatability of coarse particles.
To analyse the floatability of coarse particles, it is convenient to consider each one of the successive steps required to accomplish flotation: collision, adhesion, formation of an aggregate stable in the hydrodynamic system, levitation, transfer to the froth phase and finally permanence in the froth phase. One or more of these steps can be a limiting factor in the flotation of coarse particles.
All theoretical and experimental data indicate that the collision efficiency is favored by increasing particle size, so this factor can be discarded as a serious limitation to the recovery of coarse particles.
The probability of adhesion (or attachment) is determined by the surface hydrophobicity and by the induction time. Hydrophobicity should not be affected by the particle size and it is clear that highly hydrophobic coatings (contact angles of 50-60 degrees or more) can be obtained with conventional flotation reagents. On the other hand, some authors have proposed an increase in induction time (therefore, a decreased adhesion) with increasing particle size, but no explanation is offered about the reasons for such an increase. There is also reports derived from theoretical models that indicate that increasing the bubble size decreases the efficiency of collection.
According to theoretical developments, the floatability of particles is determined by the balance of the forces acting on the particle-bubble aggregate including: weight in the gravitational field, buoyancy, hydrostatic pressure, and capillary, tension, compression and shear forces prevailing in the hydrodynamic system. In the absence of turbulence, particles much larger than 1.0 mm should float if the contact angle is 50-60 degrees. In a turbulent field, the upper grain size limit in flotation would be reached when the kinetic energy of the particle (determined by its velocity and weight) is larger than the energy required for detachment. For a given contact angle, the upper floatable size limit would be drastically reduced in a turbulent field to one half or less than the equivalent value under quiescent condition. There is some experimental evidence that supports this turbulence determined disruption mechanism.
Once a stable particle-bubble aggregate is formed, under quiescent conditions, the flotation of large particles is limited by the buoyancy factor. A simple geometrical calculation shows that this factor is strongly dependent on bubble size. Large particles cannot be levitated by small bubbles such as those prevailing in agitated mechanical cells. It is reported that the average bubble size in most mechanical cells is about 0.5 mm. The buoyancy of spherical bubble of this size in water is just 0.065 mg.. Therefore, to float a particle of say 24 mg. (i.e. a 2.0 mm cubic particle of density 3.0 and actual weight in water of 16 mg.), the attachment of at least 247 bubbles would be necessary; this is physically impossible since, on the 24 mm.sup.2 of surface available, only about hundred 0.5 mm could be packed. On the other hand, 2.0 mm bubbles have a buoyancy of 4.18 mg.. While the particle could accommodate up to six such bubbles on its surface, just four bubbles are required for levitation. It seems clear from this analysis that any attempt to improve coarse particles flotation should address the problem of stabilizing bubbles larger than those obtained in conventional flotation machines. Regarding the transfer of the particles to the froth phase and their stability in the froth bed, it has been suggested that coarse particles destabilize the froth and it is well established that coarse particles drain back to the cell faster than small particles.
Conventional mechanical flotation cells are required to perform two inherently contradictory tasks: first to provide enough agitation to create the turbulence level necessary to suspend the particles, disperse the air and promote the subsequent bubble-particle collision and, secondly, to provide quiescent hydrodynamic conditions to avoid disruption of the particle-bubble aggregate and also avoid the transfer of gangue to the froth layer. These tasks can be accomplished fairly well by most mechanical cells when not too coarse or not too fine particles are treated. However, when the flotation of coarse particles is intended, a higher level of turbulence is required to keep the particles from settling; but, at the same time, less turbulence is required to stabilize larger bubbles and to account for the larger inertial forces that can more easily disrupt the particle-bubble aggregate.
Non-mechanical cells present the advantage of lower turbulence which implies a more stable particle-bubble aggregate and the possibility of larger bubbles. However, the transfer of particles to a froth phase remains a problem, particularly in column-type cells where the wash water flow is likely to be an additional barrier for coarse particle recovery. One solution to the problem of transfer to a froth bed is the skin flotation method as practiced in different variations in phosphate rock processing. This method, however, presents the inconvenience of low capacity and difficult control.